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Dong Yang, Zhiqin Xi, Qiang Chen, Shuisheng Li, "Mechanical Properties of Tunnel Muck with Fly-Ash Geopolymer", Advances in Civil Engineering, vol. 2020, Article ID 7247134, 10 pages, 2020. https://doi.org/10.1155/2020/7247134
Mechanical Properties of Tunnel Muck with Fly-Ash Geopolymer
The tunnel muck has a great potential to be used as a recyclable engineering material in transit and other civil work projects instead of being deposited as construction waste. In this work, the geopolymer is generated by alkali-activated fly ashes, which act as cementitious materials to strengthen the tunnel muck. The tunnel muck has to be dehydrated, grinded, and screened before being treated by alkali-activated fly ash. The effect of the mass ratio between fly ash and tunnel muck (Mfa/Mtm), the mass ratio between Na2SiO3 solution and NaOH solution (), the ratio between liquid and solid (Mliquid/Msolid), and molarity of NaOH on the strength of geopolymer were systematically studied by conducting the uniaxial compression experiments. The experimental results indicate that the liquid-to-solid ratio is the most important parameter to the geopolymer strength after the alkali-activated fly ash treatment. On the contrary, molarity of NaOH is less effective on the geopolymer strength. Moreover, the optimum scheme is concluded according to the experimental results as follows: the mass ratio between tunnel muck and fly ash, the mass ratio between Na2SiO3 solution and NaOH solution, the ratio between liquid and solid, and molarity of NaOH are 1 : 2, 1.8, 0.18, and 10 mol/L, respectively. Meanwhile, the SEM images indicate that flocculence from the active substance in fly ash is a crucial component as the cementing material.
Tunneling structures have been well applied in urban subway construction and traffic tunnel as one of the most effective ways to enhance the urban land utilization efficiency [1, 2]. People are even proposing to apply the tunneling structures into the Lunar exploration [3–5]. Meanwhile, a big amount of excavated materials/tunnel muck has been removed from tunnel, which has been considered to be waste material of little or no value for a long time [6, 7]. The main contents of the tunnel muck are gravel, sand, and clay with different amount of water. The instability of these geomaterials might trigger dreadful disasters [4, 8–12]. Hence, an efficient way to deposit or even to recycle the tunnel muck is of great significance to the environment.
The gravel and sand are typical geogranular materials [13–16], whose mechanical properties have been systemically reported by using the photoelastic experimental method since the 1990s [17–21], and they can be very easily used as building materials after being filtered out from the tunnel muck. However, the clay content in the tunnel muck is always one of the trickiest parts to deal with, which is usually mixed with ameliorants like bentonite, frothier, resin, tackifier, and so forth. Efforts have been made to recycle the clay content into geopolymer as a green material all the time [22–24]. Davidovits  presented a new type of material called geopolymers as a possible solution to this problem, since their physical properties make them a viable alternative for many conventional cements and plastics. Compared to conventional cements, geopolymers are powerful with high chemical resistance, fire resistance, temperature resistance, and so forth . In view of these superior properties, generating geopolymers by using alkali-activated fly ash to improve the bearing capacity of foundation soil has been reported recently [27, 28]. Moreover, geopolymers combined with loess and fully weathered granite were developed as useful building materials [29–31]. However, little literatures can be found on the alkali-activated fly ash as cementing material for tunnel muck.
In this work, the geopolymer that is generated by alkali-activated fly ash for reusing the high clay content tunnel muck is studied. For making the geopolymer samples, the advantages and disadvantages between casting method and pressing mold method are discussed. In order to understand the strength of the geopolymer sample, the orthogonal analysis is conducted based on the compression experiments by varying four different parameters. The optimum scheme to make the geopolymer sample from the high clay content is proposed eventually, which could be a valuable reference for the tunnel muck reutilization.
2. Experimental Materials and Protocol
2.1. Experimental Materials
The tunnel muck excavated from subway tunnel in Changsha is used to make the geopolymer. The main content of the tunnel muck here is completely weathered conglomerate with approximately 26.9% moisture content, 26.9% silt content, and 18.4% clay content. The particle size distribution of the tunnel muck is shown in Figure 1. The measured plastic limit and liquid limit of the excavated tunnel muck are 13.3% and 29.0%, respectively.
Before fabricating the geopolymer, the excavated tunnel muck is dried and smashed. The fine grain muck with less than 2 mm diameter is picked by sieve. The X-ray fluorescence (XRF) results of a typical fine grain muck after filtering by 2 mm diameter sieve are shown in Table 1. Most of the elements are SiO2 and Al2O3 of the fine grain muck. There is approximately 1 percent of MgO and very few CaO in the fine grain muck. The fly ash with 450 m2/kg specific surface area is applied into the tunnel muck in this work. The alkali-activator is a mixture of NaOH solution and Na2O·nSiO2 in this work, where the NaOH solution is made by dissolving the industrial grade tablet alkali into water. The freshly prepared NaOH solution has to be cooled down to the room temperature before being used to make the geopolymer. In order to ensure equilibrium state in the mixture, the mixed solutions need to be stored for a minimum of 24 hours prior to use as suggested by Duxson et al. [29, 32, 33].
2.2. Experimental Protocol to Mode the Geopolymer Sample
Four different methods were tried to make the raw materials mixtures. The one that can have the highest strength sample is applied in this work. The details of the applied method include the following: (1) add a certain amount of muck in the container; (2) according to the scheme, the fly ash is well mixed with muck; and (3) the solution is poured into the muck and fly ash mixture, and well mix them. The details of the other three methods will not be introduced here, since these methods are not applied in the present work. Two prevailing procedures can be applied to make the geopolymers: casting and mold pressing (semidry pressing) . By using casting method, the raw material mixtures with alkali-activated fly ash are made as flexible slurry is poured into sample mold. The geopolymers will form after a certain time. The casting method is a relatively easy method, which can even make complicated geometry geopolymer samples. Different amount of water is added into the sample to get the optimal amount of water that can make the raw material flexible. In order to pour the raw materials into the mold, a mass percentage of approximately 20%∼40% of water has to be added, which is too much. Moreover, there will always be cracks forming on the surface of geopolymer sample due to the high clay content as shown in Figure 2(a), [35, 36]. For the mold pressing method, the raw materials mixtures with alkali-activated fly ash are made as colloidal solution to be compressed to solid sample in the mold, which requires much less water than casting method. The force applied here is 19.625 kN with a 10 MPa corresponding pressure. Moreover, the same pressure (10 MPa) is used to make the samples with pressing method in this work. The geopolymer sample made by mold pressing method has relatively smooth surface as shown in Figure 2(b). Hence, the mold pressing method is applied in this work to make the geopolymers.
2.3. Uniaxial Compression Protocol
Comprehensive experiments are conducted to systemically study the effect of each parameter on the strength of geopolymers. Orthogonal experimental design is applied to schematize the uniaxial compression protocol. Four different parameters are considered as orthogonal factors, and three factor levels are involved in total. The details of the experimental protocol are shown in Table 2. The parameters include the mass ratio between fly ash and tunnel muck (Mfa/Mtm), the mass ratio between Na2SiO3 solution and NaOH solution (), the mass ratio between liquid and solid (Mliquid/Msolid), and molarity of NaOH, where the mass ratio between fly ash and tunnel muck means the mass of fly ash versus the mass of dry tunnel muck here.
The geopolymer sample is pressed in a cylinder mold with a diameter of 50 mm and a height of 100 mm under 10 MPa compression pressure. The samples are cured for 28 days before being placed on the uniaxial compression apparatus (Instron 1346). The experiments are conducted by combining stress control method and displacement control method. At the initial state, the stress control method is applied with 10 kN/min loading speed, and the 0.2 mm/min displacement control method is applied at 80% of the estimated peak strength until complete failure stage.
3. Experimental Results
3.1. Stress-Strain Curve of the Geopolymers
The stress-strain curves of all 9 × 3 uniaxial compression experimental results are shown in Figure 3. In general, four stages can be observed in the stress-strain curves; the region of each stage on a typical stress-train curve is shown in the inset of Figure 3. The four stages include the following. (1) The first is strain hardening stage, which happens right after the initial compression. The slope of the curve tends to increase as the compression strain increases, which indicates the vertical stiffness is increasing in this stage. (2) The second is linear elastic deformation stage, at which the stress increases linearly as the strain increases. The deformation of the geopolymer sample is recoverable after unloading in this stage. No or only a few local rearrangements happen in the sample. The maximum stress in the linear elastic deformation stage is approximately 50% of the peak stress. (3) The third is strain softening stage, where the slope of the stress-strain curve decreases as the strain increases. Cracks start to form in the sample and eventually percolate through the sample. Meanwhile, the stress is corresponding to the peak stress. (4) The fourth is post-peak-deformation stage, where the stress decreases as the strain increases. Several vertical cracks along the compression direction would be generated with the increase of strain.
3.2. Mechanical Properties of the Geopolymer Samples
Several mechanical parameters of the geopolymer samples can be extracted from the stress-strain curves, including strength, elasticity modulus, residual strength, peak strain, and tenacity. The elasticity modulus is calculated by fitting the stress-strain curves in the range from 25% to 50% of the compression strength. For the definition of residual strength, the strain at peak stress, γp, is obtained from the stress-strain curve first, and then the stress at the strain equaling 2γp is defined as residual strength. The stress usually reaches a stable state after the strain becomes twice bigger than that at the peak stress. Hence, the stress at the strain twice bigger than that at the peak stress is defined as the residual strength. The tenacity is defined as the area under the stress-strain curve. The mechanical properties of the geopolymer samples from uniaxial compression experimental results are shown in Table 3, where all the mechanical parameters are averaged over three experiments with the same initial condition.
As shown in Table 3, group 7 sample has the maximum strength, which is 26.41 MPa. Group 4 has the highest elasticity modulus of 4.40 GPa. The maximum residual strength is 3.88 MPa, which is exhibited by group 3. The peak strain of the tested geopolymer samples is located in 6.31 × 10−3∼9.82 × 10−3, which is several times bigger than the peak strain of concrete . The average residual strength is approximately 11.9% of the peak strength.
The orthogonal analysis is applied to understand the importance of each parameter to the strength and tenacity of the geopolymer sample [38, 39]. The details of orthogonal analysis are shown in Table 4. The results indicate that the liquid/solid ratio is the most important parameter to the geopolymer strength after the alkali-activated fly ash treatment. On the contrary, the geopolymer strength is less sensitive to molarity of NaOH. Meanwhile, the optimum scheme is concluded according to the experimental results as follows: the mass ration between fly ash and tunnel muck, the mass ratio between Na2SiO3 solution and NaOH solution, the ratio between liquid and solid, and molarity of NaOH are 1 : 2, 1.8, 0.18, and 10 mol/L, respectively.
The effects of the mass ratio between fly ash and tunnel muck (Mfa/Mtm), the mass ratio between Na2SiO3 solution and NaOH solution (), the ratio between liquid and solid (Mliquid/Msolid), and molarity of NaOH on the strength of geopolymer are shown in Figures 4(a)–4(d). The strength approaches the highest value when Mfa : Mtm = 1 : 2 and molarity of NaOH is 10 mol/L. The strength increases as the Mliquid/Msolid increases, which is one of the most important control factors to the geopolymer strength. However, it is hard to make the geopolymer sample with Mliquid/Msolid > 0.2 as the mixture would be squeezing out from the mold. The ratio Mliquid/Msolid = 0.18 is set to make the geopolymer in this work. is another important factor to the geopolymer strength, which has a negative correlation with the geopolymer strength. The tenacity of the geopolymer sample affected by these four influence factors is shown in Figures 4(e)–4(h). According to the orthogonal analysis results in Table 4, the geopolymer sample has the highest tenacity at Mfa : Mtm = 1 : 3, = 1.8, Mliquid/Msolid = 0.8, and molarity of NaOH is 12 mol/L.
Microscale pore filled by the agglutination aggregates is another important factor that can influence the strength of geopolymer sample. The filling degree can be estimated by the density of geopolymer sample, which has positive correlation with the density. Figure 5 shows the strength of the geopolymer sample as a function of density for different mass ratio between fly ash and tunnel muck. The strength increases as the density increases when Mfa : Mtm = 1 : 2 and 2 : 3. However, when Mfa: Mtm = 1 : 3, the strength increases to approximately 20 MPa as the density increases and then decreases.
The scatter of strength and tenacity versus elasticity modulus and peak strain is shown in Figure 6. The peak strain seems to have an asymptotic value of 8 × 10−3, which is indicated by the dashed lines in Figures 6(c) and 6(d). Both of elastic modulus and tenacity show positive correlation with strength, while the strength () and elasticity modulus () can be fitted by a linear equation as
The pictures of failure geopolymer sample are shown in Figure 7. A cone geometry part can be found for almost every failure sample, which might be due to the basal friction between the machine and sample. Meanwhile, an “X” shape shear band geometry can also be found in the failure sample as shown in Figure 7(g) [16, 40]. There is another uncommon failure as shown in Figure 7, where the top of the sample is relatively complete, while the bottom of the sample is broken into pieces. This might be caused by the inhomogeneous density distribution in the sample, which results from the high friction between the sample and the mode stopping the sample from sliding down under compression applied from the top . A much dense top and less dense bottom would form in this case.
Different microscopic levels of scanning electron microscope (SEM) tests were conducted to understand the microscope structure of the geopolymer. The SEM images are shown in Figure 8, the flocculence is geopolymers, spheres are fly ashes, and bulks are soil particles. The fly ashes and soil particles are wrapped by flocculence (Figures 8(a) and 8(b)) to strengthen the sample. The flocculence is mainly from the chemical reaction of active substance in fly ashes. The left materials without chemical reaction became filler and skeleton of geopolymer sample, as shown in Figure 8(c). The corrosion on the surface of fly ash particle can be seen after the chemical reaction; several structural holes are visible as shown in Figure 8(d).
The mechanical properties of the geopolymer made by different recipes are reported. The strength, tenacity, and residual strength are analyzed systematically. The orthogonal analysis is applied to understand the effects of the mass ratio between fly ash and tunnel muck (Mfa/Mtm), the mass ratio between Na2SiO3 solution and NaOH solution (), the ratio between liquid and solid (Mliquid/Msolid), and molarity of NaOH on the strength of geopolymer. Our experimental study has made the following findings:(1)The pressing mold method is better than casting method for the tunnel muck with high clay content. In order to make high-strength geopolymer sample, the dry soil and fly ash should be mixed homogeneously together first and then mixed with alkali-activator.(2)The orthogonal analyses conclude that the optimum scheme for making geopolymer sample is to set the mass ration between fly ash and tunnel muck, the mass ratio between Na2SiO3 solution and NaOH solution, the ratio between liquid and solid, and molarity of NaOH to be 1 : 2, 1.8, 0.18, and 10 mol/L, respectively.(3)The failure of geopolymer sample shows cone geometry part and “X” shape shear failure. The SEM images show that the flocculence from the active substance in fly ash is a crucial component as the cementing material.
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This work was supported by the China State Construction Engineering Corporation Limited Funded Project (Grant no. CSCEC-2018-Z-1), Natural Science Foundation of Hunan Province, China (Grant no. 2018JJ3578), and China Construction Fifth Engineering Division Group., LTD, Funded Project (Grant no. CSCEC5b-2017-03).
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